1. Field of the Invention
The present invention relates to a high-frequency multielement array planar antenna, and more particularly to a multielement planar antenna which employs microstrip lines for improved sensitivity and has active devices and integrated circuits (ICs) mounted thereon for use as an active antenna.
2. Description of the Related Art
Planar antennas have widely been used as microwave and millimeter wave antennas in the fields of radio communications and satellite broadcasting applications. Generally, planar antennas include microstrip line antennas and slot line antennas. Of these planar antennas, microstrip line antennas find wider use than slot line antennas because they have a simpler feeder structure and better radiating characteristics. However, since the microstrip line antennas have a low antenna gain, it is customary for them to be constructed as a multielement array antenna having a plurality of antenna elements.
FIG. 1 is a plan view of a conventional multielement planar antenna. As shown in FIG. 1, the conventional multielement planar antenna has substrate 1 made of a dielectric material and a plurality of circuit conductors, each functioning as an antenna element, disposed in a matrix on one main surface of substrate 1. In FIG. 1, the multielement planar antenna has a matrix of four antenna elements 2a through 2d each comprising a rectangular circuit conductor. A ground conductor in the form of a metal conductor is disposed on the other main surface of substrate 1. The antenna elements and the ground conductor jointly make up a microstrip line resonator in coaction with an electric field generated between the two main surfaces of substrate 1 and a magnetic field generated due to the electric field. Feeding lines are connected to respective antenna elements 2a through 2d to form the planar antenna. The antenna frequency of transmission and reception in the planar antenna generally correspond with the resonant frequency of the microstrip line resonator.
The feeding lines are disposed on the one main surface of substrate 1, and cooperate with the ground conductor on the other main surface of substrate 1 in forming microstrip lines. The feeding lines extend from a feeding end and are branched and connected in parallel to the respective antenna elements for thereby feeding the antenna elements through a parallel in-phase branched structure. Specifically, antenna elements 2a, 2b are connected to each other by feeding microstrip line 3a, and antenna elements 2c, 2d are connected to each other by feeding microstrip line 3b. Feeding Microstrip lines 3a, 3b are connected to each other by feeding microstrip line 3c, which is connected to feeding microstrip line 3d connected to the feeding end. To achieve impedance matching between the feeding microstrip lines, matching circuits 4a through 4c are incorporated in respective regions where a feeding microstrip line is branched into two feeding microstrip lines. Matching circuits 4a through 4c are arranged as wider regions of some feeding microstrip lines.
Since the feeding circuit is essentially of a parallel in-phase branched circuit on the conventional microstrip line planar antenna with plural antenna elements, antenna elements 2a through 2d are required to be fed in the same direction at all times. Therefore, the sum of the lengths of feeding microstrip lines 3a through 3d is basically large, and additionally large because of matching circuits 4a through 4c. In FIG. 1, each of antenna elements 2a through 2d is fed from an upper side of the rectangular circuit conductor thereof. However, each of antenna elements 2a through 2d may be fed from a lower side, a left side, or a right side of the rectangular circuit conductor thereof insofar as it is fed in the same direction.
Furthermore, since impedance matching at the branched points of the feeding microstrip lines is indispensable, matching circuits 4a through 4c tend to cause a large feeding loss. As antenna elements 2a through 2d are energized in the same direction, the feeding lines need to be positioned around antenna elements 2a through 2d, resulting in a likelihood of interference between the antenna elements and the feeding lines. For these reasons, it is difficult to reduce the size of the multielement planar antenna of the type described above, and the multielement planar antenna fails to provide sufficient electric characteristics including sensitivity, directivity, and the like.
A microstrip line structure is an unbalanced transmission line in which a conductor pattern and a ground conductor, which serve as an electric pair, are disposed respectively on one and other main surfaces of a substrate. Even if attempts are made to mount circuit devices such as ICs including bare chips, MMICs (monolithic microwave integrated circuits.) and the like, and antenna elements integrally on one main surface of the substrate of a microstrip line planar antenna thereby to produce a transmission/reception module, a complex connection structure is needed to connect ground terminals of the integrated circuits and the MMICS to the ground conductor. The complex connection structure makes it impossible to employ a bare chip mounting process based on the bump technology and a surface mounting process for packaged ICs.